TCP协议的应用，实现一个服务器处理多个客户端请求（向服务器发送消息，服务器接收到后以原消息返回给客户端。）

客户端类

import java.io.*;import java.net.Socket;import java.util.Scanner;//先写出后读入public class EchoMultiClient {    public static void main(String[] args) {        Scanner input = new Scanner(System.in);        try {            //创建一个Socket对象，指定要连接的服务器            Socket socket = new Socket("127.0.0.1",6666);            //向服务器发送数据，先从socket获得输出流，并包装            PrintStream ps = new PrintStream(new BufferedOutputStream(socket.getOutputStream()));            System.out.println("请输入要发送的内容：");            String info = input.nextLine();            ps.println(info);            ps.flush();            //读取服务器返回的消息，先从socket获得输入流，并包装            BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));            info = br.readLine();            System.out.println(info);            //关闭流            br.close();            ps.close();        } catch (IOException e) {            e.printStackTrace();        }    }}

服务器类

import java.io.*;import java.net.ServerSocket;import java.net.Socket;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/*** 处理多个客户端* 主线程用于监听客户端的连接，每次有链接成功，开启一个线程来该客户端的消息*/public class EchoMultiServer {    public static void main(String[] args) {        //创建一个固定大小的线程池        ExecutorService es = Executors.newFixedThreadPool(3);        try {            //创建服务器端套接字，并指定端口号            ServerSocket serverSocket = new ServerSocket(6666);            System.out.println("服务器已启动，等待连接。。");            //现在不直接处理了，主线程只负责接待，具体操作交由其他线程实现            while (true) {                Socket socket = serverSocket.accept();                System.out.println("一个客户端连接成功。。。" + socket.getInetAddress().getHostAddress());                es.execute(new UserThread(socket));            }        } catch (IOException e) {            e.printStackTrace();        }    }}/*** 用来处理客户端请求的线程任务*/class UserThread implements Runnable {    //处理谁？    private Socket socket;    public UserThread(Socket socket) {        this.socket = socket;    }    @Override    public void run() {        try {            //读取客户端请求            BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));            String info = br.readLine();            System.out.println(info);            //向客户端返回消息            PrintStream ps = new PrintStream(new BufferedOutputStream(socket.getOutputStream()));            ps.println("echo:" + info);            ps.flush();            ps.close();            br.close();        } catch (IOException e) {            e.printStackTrace();        }    }}